The present invention relates to an apparatus for manufacturing magnetic plastic rotor disks, and more particularly to such an apparatus suitable for production of flattened columnar magnetic plastic rotor disks to be used as meter rotors, for example.
Such a magnetic plastic rotor disk is conventionally used for a vehicle meter such as a vehicle speed meter and an engine speed meter for indicating rotating speeds corresponding to a vehicle speed and rotational frequency of the engine thereof. FIG. 4 shows a magnetic plastic rotor disk b having a central rotary shaft a and magnetized in one direction perpendicular to the rotary shaft a to have an N-pole and an S-pole at diametrically opposite positions. The magnetic plastic rotor disk b is required to have a characteristic as shown in FIG. 5 wherein distribution of a magnetic flux density on an outer circumferential surface of the magnetic plastic rotor disk b varies sinusoidally with respect to a rotary angle of the magnetic plastic rotor disk b.
FIG. 6 shows a conventional apparatus for manufacturing such a magnetic plastic rotor disk. Referring to FIG. 6, a magnetic plastic material A containing a plastic matrix and a magnetic powder is injected from an injection device B through a sprue and runner D formed in a two-plate side gate type mold C into a plurality of cavities E. A reference numeral F designates a yoke around which a coil (not shown) is wound. The yoke F is so located as to surround a parting surface C' of the mold C, so that a magnetic field is applied to the magnetic plastic material A injected into the cavities in one direction as shown in FIG. 7 to thereby magnetize the magnetic powder contained in the material A so that the particles of the magnetic powder are magnetized in the same magnetic orientation.
However, since this conventional magnetic plastic rotor disk manufacturing apparatus is of a side gate type having two plates, a gate burr remarkably appears on the external surfaces of the magnetic plastic rotor disk after molding, causing turbulence of the distribution of the surface magnetic flux density of the magnetic plastic rotor disk. As a result, a desired characteristic of the magnetic plastic rotor disk cannot be obtained. Accordingly, it is necessary to conduct a deflashing process after a gate cutting process. Further, if the magnetic plastic rotor disk is in a magnetized condition in the deflashing process, small flash particles generated in the deflashing process will be magnetically attached to the rotor disk. Therefore, it is necessary demagnetize the rotor disk before the deflashing process and then magnetize the same again after the deflashing process. Thus, many troublesome processes are necessary after molding in the prior art device, causing an increase in production cost.